Excitons, which are electron-hole pairs bound by the Coulomb attraction, can form Bose-Einstein condensates (BEC) at low temperatures. By placing electrons and holes in spatial separated layers, double-layer electronic system prevents exciton recombination and enables exciton condensation in equilibrium. In this talk, I will present our study of exciton condensation in graphene double-layer heterostructure under strong magnetic fields. Fascinating properties of exciton condensates such as interlayer coherence and exciton superfluidity are manifested by quantized Hall drag and vanishing counterflow resistance. More interestingly, the pairing strength of interlayer excitons can be tuned by exciton denisty. When the densities are low, strong Coulomb attraction organizes all electrons and holes into real space pairs, which establish BEC condensates. In contrast, at high densities, the interlayer Coulomb interaction is weakened by screening and can only induce momentum-space pairing near Fermi surface, similar to a BCS superconductor. By changing the exciton density, our experiments demonstrate the BEC-BCS crossover of exciton condensation. Furthermore, we also observed signatures of Berezinskii–Kosterlitz–Thouless (BKT) transition from current (I)-voltage (V) relation in the BCS regime.